1. Field of the Invention
The present invention relates to metallurgy and more particularly to a process and apparatus for the continuous casting of metal in an electromagnetic field. The invention permits a wide choice of metals to be used for casting ingots by the proposed method.
This invention may find most utility in the production of ingots by continuous and semi-continuous casting processes wherein a magnetic field is used for forming the ingot liquid portion in an electromagnetic field in the event of casting ingots from refractory and easily oxidizable metals and alloys which do not form sufficiently protective oxide films on the melt surfaces thereof as well as from alloys composed of high vapour-pressure alloying components. In addition, the invention is readily applicable in the production of ingots, effected by means of remelting consumable electrodes.
2. Description of the Prior Art
For example, U.S.S.R. Inventor's Certificates No. 338,037 and No. 282,615 describe processes and apparatus for continuous casting of metals in the electromagnetic field of a magnetic inductor functioning as contactless means for forming the ingot liquid portion, with the side surface of the ingot being subjected to direct and intensive cooling.
The practice of casting ingots in electromagnetic field from aluminum and some of its alloys has been found superior to conventional continuous casting process performed on a continuous casting machine provided with a slidable force-cooled mold. The ingots are produced to have high-quality side surfaces and a uniform chemical composition across its section, as well as uniform crystalline structure, the features substantially improving the ingot workability and mechanical properties of alloys.
It should be observed, however, that the prior-art apparatus and processes of casting metal in electromagnetic field permit the production of quality ingots which are cast from the metals and alloys which form on their surfaces a uniform and dense protective oxide film similar to that formed in the process of casting ingots from aluminum, which film makes it possible for the ingot liquid portion to be supported in the form of a column, with the static pressure of the ingot metal being slightly increased or the process conditions, such as the speed of lowering the bottom plate with an ingot, shocks, the bottom plate vibration, the rate of solidification, being slightly varied in the course of the ingot casting and solidification process.
There are known several types of high-temperature metals, as well as high-alloyed metals containing highly volatile components. Where such metals are subject to casting in electromagnetic field, violent turbulence takes place in the ingot liquid zone, caused by high convective flows of melt in the ingot liquid zone and by vertical uplift of the bubbles due to the sublimation of the alloying components. If not damped, such violent turbulence on the surface of the ingot liquid portion, as well as the discharge of slag and oxide inclusions, causing nonuniform interaction with magnetic field, impair the process of formation and solidification of the side surface of the ingot liquid portion, thereby making it impossible to produce high-quality ingots. Furthermore, because of the violent turbulence in the ingot liquid zone, and in its top portion in particular, accurate control over the level of the ingot liquid surface is rendered difficult to carry out in the event of casting ingots from the above-mentioned alloys. It is known that a change in the level of the ingot liquid zone brings about a proportional change in the ingot cross-sectional dimensions. With an excessive height of the ingot liquid zone, the casting process is disrupted. The aforementioned features, specific to the prior-art continuous casting processes and apparatus, are aggravated and become harmful in the casting of heavy nonferrous and ferrous metals and alloys thereof, which form no protective oxide film on the melt surface, making it possible for the column of the ingot liquid zone to be supported under the action of an electromagnetic field, which is otherwise spread over a more than 20 percent increase in the height of the ingot liquid zone.
It is therefore necessary to minimize detrimental effect of the ingot liquid zone turbulence on the ingot formation and solidification process, and to prevent oxide film and foam from setting onto the ingot side surface.
The aforementioned disadvantages of the prior-art casting apparatus and processes are mostly due to high sensitivity of the ingot forming process to slight variations in the process conditions. Thus, a high sensitivity of the contactless process of the ingot formation effected under the action of electromagnetic field is regarded as one of the basic difficulties encountered in the course of practical implementation of the known casting process which turns out to be impractical where high-quality ingots from refractory and easily oxidizable metals and alloys, not forming sufficiently protective oxide film on the melt surface, are required.
The above-mentioned disadvantage of the prior-art casting process is due to the difficulty of ensuring constant control of the resultant magnetic field forming the ingot liquid portion, and of the metal static pressure acting vertically on the ingot liquid portion, as well as due to the absence of low-inertion automatic correction of the ingot forming process when introducing variations into the process conditions in the course of casting high-temperature metals and alloys.
It has been found that a mere increase in the height of the ingot liquid portion, for example, by at least 3 to 5 mm, brings about respective increase in the cross-sectional dimensions of the ingot liquid portion. The casting process is disrupted as the balance of forces between the metal static pressure and that of magnetic field is violated to exceed the permissible level. Variations in the height of the ingot liquid portions or in the electric parameters of the magnetic pumping means, as well as variations in the ingot withdrawing speed, adversely affect the quality of ingots cast from heavy high-temperature and easily oxidizable metals and alloys, such as aluminum-base alloys, which do not form sufficiently protective oxide films on their melt surfaces, ensuring stable ingot-forming process.
Metals such as aluminum and some of its alloys do not require good heat protection or protection from oxidation of the ingot liquid portion, since the oxide film formed on the ingot liquid portion serves as a reliable protection from oxidation and, consequently, prevents the formation of slags and froth-like oxides on the surface of the metal, even if slightly overheated prior to casting operation.
The metals and alloys, having relatively high melting and solidification temperatures and being easily oxidizable, do not tend to form such oxide film on their melt surfaces as aluminum and some of its alloys. Moreover, such types of metals tend to form on their surfaces a thin skin of metal solidifying on the meniscus of the ingot liquid zone, which is broken by convective flows of the melt and is then entrained together with the slag and oxide solid inclusions to be transferred to the side surface of the ingot liquid portion, thereby impairing the ingot forming and solidifying process.
Attempts have been undertaken to use inert gases as protective atmosphere above the surface of the liquid portion of an ingot formed in magnetic field. For example, the U.S.S.R. Inventor's Certificate No. 455,794 describes an apparatus for casting metal in magnetic field. The apparatus is provided with a cover which closes the ingot-forming cavity and has a pipe for a protective gas to be applied therethrough. To prevent the atmosphere air from penetrating into the ingot-forming cavity, a funnel-shaped element is fixed below the water supply level and is filled with the water flowing off the surface of the water-cooled ingot and forming a steam blanket between the supplied inert gas and ambient atmosphere.
The apparatus described above allows only inert gases to be used as the protective atmosphere.
However, it is likewise impossible to ensure the production of ingots with high-quality side surface from alloys the components of which have relatively low boiling point and, therefore, with which it is preferable to have their liquid surfaces protected with flux melts. The use of flux melts with this type of apparatus is impossible because of the fact that such melts will flow off the ingot horizontal liquid surface onto the side surface thereof to interact with the cooling water. Since the apparatus cannot be hermetically sealed, it does not allow the use of vacuum.
From the above it follows that the prior-art apparatuses and processes for continuous casting of metal in electromagnetic field do not permit, on account of characteristic features inherent in the procedure of contactless formation of the ingot liquid portion in magnetic field and intensive direct cooling of the ingot side surface, the production of high-quality ingots from high-temperature and easily oxidizable metals and alloys which do not permit sufficiently protective dense oxide film to be formed on the ingot liquid surface as, for example, aluminum-base alloys, as well as from alloys having high vapour-pressure components included therein.
The prior-art casting processes in question fail to provide necessary protection to the upper portion of the ingot liquid zone from undesired losses of heat; the side surface of the ingot liquid portion being left unprotected from penetration of slag or oxide films with solid metal inclusions from the upper portion of the ingot liquid zone. This results in the impairment of appropriate conditions required for uniform formation of the ingot liquid portion and disturbs uniformity in the ingot solidification at the side surface thereof.
Furthermore, it is impossible to protect the entire surface of the ingot liquid portion with a layer of protective-degassing flux or to provide protective rarefied atmosphere thereabove.
Where ingots are cast from an alloy with high vapour-pressure components, for example, zinc in brass, the escape of vapours through the open surface of the ingot liquid portion impairs the ingot forming process accompanied by violent turbulence of metal in the ingot liquid portion and results in the appearance of flaws on the ingot side surface and its peripheral layer.
The known casting processes of the type described above fail to provide for the production of presized ingots, or ingots with the cross-sectional profile thereof being different from the cross-sectional profile of the ingot liquid portion.
Such processes are unsuitable for the production of ingots having on their side surfaces a layer of metal with chemical composition thereof being different from that of the ingot metal, for example, a layer of solidified flux protecting the ingot surface from oxidation, or a layer of clad metal, or else a thin layer of alloy, for example, copper tin, copper-lead on copper ingot.
The disadvantages inherent in the prior-art casting processes and apparatuses make it impossible to combine a highly efficient process of casting high-quality ingots in magnetic field with a casting process effected by means of melting consumable electrodes.
However, the problems posed by general and metallurgical engineering demand urgent solutions required to further improve the known processes and apparatuses for casting metal in magnetic field. The expected solutions to these problems have enormous practical significance for the production of ingots from refractory easily oxidizable metals and alloys thereof, such as iron, nickel, titanium, copper, silicon, germanium, as well as from the alloys containing high vapour-pressure components, such as, for example, zinc and aluminum, i.e. from the metals and alloys which, unlike aluminum, do not permit a sufficiently dense protective oxide film to be formed on the surface of the liquid portion of the ingot solidifying under the action of an electromagnetic field.